1. Technical Field
The invention relates to an improved ski, especially a ski having asymmetric structure.
2. Prior Art
As is known, in cross section, a ski may comprise three essential parts, namely, in order:
a first, lower assembly, consisting of a planar running sole intended for sliding, optionally bordered by metallic edges, and of at least one lower longitudinal reinforcement element, the assembly having a plane of symmetry perpendicular to the plane of the running sole on passing through the longitudinal mid-axis of this lower assembly; PA1 a second, upper assembly constituting the top and the sides; PA1 finally, an intermediate filling space contained between the first and second assemblies, in which the neutral fiber (FN) is located and which includes another reinforcement element. PA1 a first, lower assembly, including a planar running sole having a plane of symmetry P perpendicular to the plane of the running sole passing through its longitudinal mid-axis, PA1 a second, upper assembly forming a top and sides, PA1 these two assemblies defining an intermediate space, in which the neutral fiber (FN) is located and in which, at least in the region of the central support face, a principal longitudinal reinforcement element is arranged which is inclined with respect to the plane of the running sole, PA1 the ski includes a single flat strip intersecting the plane P in the region of its longitudinal mid-axis and defining two equally wide portions arranged on either side of this plane P; PA1 the flat reinforcement strip has constant inclination with respect to the plane of the running sole, close to 45.degree. over part or all of the length of the ski; PA1 the flat strip is made of metal, for example of the sawblade type, or made of a composite sheet-form laminated flat material; it may also be a piece of wood in which the stack of fibers behaves as a laminated assembly.
For a number of years, skis have been known in which the second, upper assembly assumes the shape of a shell which bears on the two lateral borders of the first, lower assembly.
As is known, a ski is subjected to several types of deformation. During strenuous activity it is mainly subjected to simple normal flexion which is exerted perpendicularly to the running sole on a ski placed flat on its running sole between two supports; this deformation allows it to match the relief of the terrain; the resistance of a ski to this deformation, that is to say its flexional stiffness or alternatively its flexibility distribution, is calculated in order to distribute the load of the skier over the snow.
A ski is also subjected to simple lateral flexion, which is exerted laterally on the side of the ski and causes bending of the longitudinal mid-axis of the lower assembly; this deformation, which is not controlled, is detrimental and impairs precise skiing. Furthermore, a ski undergoes torsion or twisting, especially at its two ends--tip and heel. As before, these deformations are parasitic and hinder correct use of the ski. Finally, a ski is subjected to composite deformations: flexion plus torsion, or alternatively normal flexion plus lateral flexion, etc.
In addition, modern skis essentially have a wide zone in the region of the tip and of the heel and a narrow zone in the region of the support face. Most generally the width of the tip is greater than that of the heel which, in turn, is greater than that of the support face. The crest line joining the tip to the heel, called the "ridge line", is an essential characteristic of a ski. It various from one manufacturer to another, in particular depending on the specific use envisaged. Thus, a slalom ski does not have the same ridge line as a downhill ski, just as that of a leisure ski is different from that of a competition ski.
This ridge line is very important when skiing. It is therefore important that, when being used, it is not altered randomly during deformations of the ski.
As is known, in a beam, the "neutral fiber" (FN) is the zone in which the compressive and tensile stresses cancel. This fictitious zone is defined as a function of the shapes and of the internal composition of the beam. Since a ski behaves as a beam, it consequently has a "neutral fiber", referred to herein below as (FN).
The mechanical reinforcements in a ski are generally located in the intermediate space, actually on either side of the neutral fiber, especially in order to ensure correct mechanical behavior. In order to optimize the efficiency of the reinforcements, it is important to position them as far as possible from the neutral fiber (FN), because it is the square of the distance from the reinforcement to the neutral fiber which is relevant when calculating flexional stiffness.
Most generally, a ski is symmetrical with respect to the plane of symmetry perpendicular to the plane of the running sole and passing through the longitudinal mid-axis thereof.
For a number of years, in order to satisfy technical and esthetic requirements, asymmetric skis have been proposed, that is to say skis whose shape and volume vary in thickness, in length and in width from one side of a given ski to the other, but in which the ridge line nevertheless remains symmetrical. In such skis, in order to conserve simple normal flexion, that is to say without lateral flexion or without parasitic torsion, the manufacturers have been constrained to design an opposed internal reinforced structure correcting the dynamic effects of the asymmetry in shape.
In fact, in order to prevent parasitic lateral flexion arising during normal flexion, it is necessary for the mechanization elements, that is to say the reinforcements, to be placed such that the flexion stiffnesses are symmetrical with respect to the vertical mid-plane passing through the longitudinal mid-axis of the running sole.
Furthermore, in document FR-A-2,611,517, corresponding to document U.S. Pat. No. 5,108,124, a ski has been described which has, against the shell and on each of its two sides, an inclined variable reinforcement which is symmetrical with respect to the mid-plane P of the running sole. When used, this ski behaves as a traditional symmetrical ski.
In document FR-A-2,687,924, the Applicant has suggested inserting, in the intermediate foam-filled space, at least one, optionally asymmetric, openworked tubular reinforcement arranged respectively to the front and to the back of the support face region. This embodiment aims to improve guiding of the ski without stiffening it, in order to make it more comfortable. However, it does not make it possible to ensure optimum transmission of bearing forces on the edges because, as is known, it is in the region of the support face that these bearing forces are most powerful.
In document FR-A-2,589,745, in order to improve strength, it has been proposed to resort to interposed pieces in order to ensure correct distribution of stresses, it being possible for these pieces to form lower or upper corner edges. This theoretical arrangement is difficult to produce, because of the non-negligible risks of detachment of these interposed pieces as a result of shearing induced by the permanent flexion stresses.
In document FR-A-2,590,179, in order to ensure correct transmission of the forces to the inner side of a cross-country ski, it has been proposed to embed, in the intermediate space and in the region of the support face, a ribbed reinforcement in the shape of an inverted U having particular architecture, the side of the long branch of which bears on the inside edge. This arrangement, which is favorable for the practice of skating steps, is not concerned with directing or controlling deformations under bearing forces, because its short length does not extend beyond the support face region, which cannot alter the behavior of the ski.
It can therefore be stated that, in the prior art hitherto known, all skis have been designed in order to avoid generating parasitic lateral flexion during deformation in normal flexion. It can thus be stated that the ridge line, that is to say the respective position of a point on the lower crest with respect to another point located to the front or to the rear thereof, retains its characteristics without modification.